In conventional metallic waveguides, the cut-off frequency for the transmission of electromagnetic (EM) waves depends on the transverse dimension of the waveguide. That is, the longer the wavelength of the guided waves, the larger the transverse dimension of the waveguide must be. Thus for long-wavelength microwave or radio waves it may not be practical to have EM waveguides since the transverse dimensions would have to be very large.
Recently, it has been shown that EM wave transmission through a silver film with a periodic array of subwavelength holes can be significantly higher than the conventional prediction. Subsequently, two possible mechanisms to realize high transmission of EM waves were identified. One is the surface plasmon (SP) resonance, which explains the Ebbesen experiments, and the other is the waveguide mode resonances inside metallic slits due to the Febry-Perot (FP) interferences. In the SP mechanism, enhanced transmission can only be achieved if the metallic film is very thin, due to the evanescent coupling. Hence such a mechanism is not suitable for waveguide considerations. In the second mechanism, involving the slit geometry, there is a fundamental TEM propagating wave mode. However, the latter requires at least one dimension of the slit cross section be comparable to the relevant wavelength, a well-known limitation for the propagation of EM wave in waveguides and resonant cavities.
Another component widely used in EM wave and electronic signal transmission is the delay line. For free space propagation of EM waves, a piece of dielectric plate can have delay functionality through which EM wave penetrates. However, such plates can delay only by a small amount due to the generally low dielectric constant of materials at high frequencies, and the limited thickness of the plate. Therefore, reducing the thickness and increasing the dielectric constant are advantageous for EM wave delay line functionality.